1. Field of the Invention
The invention relates to an overvoltage protection means for use in power supply, especially low voltage systems, with a housing, a first electrode, at least one second electrode, an arc chamber which within the housing between the electrodes, and with a breakdown spark gap between the two electrodes. In such overvoltage protection means, when a breakdown spark gap is ignited, an arc forms between the two electrodes.
2. Description of Related Art
Electrical, but especially electronic measurement, control and switching circuits, mainly also telecommunications equipment and systems, are sensitive to transient overvoltages, as can occur especially by atmospheric discharges, but also by switching operations or short circuits in power supply grids. This sensitivity has increased to the extent electronic components, especially transistors and thyristors, are being used; in particular, increasingly used integrated circuits are highly endangered by transient overvoltages.
Electrical circuits normally work without problems using the voltage specified for them, i.e., the rated voltage. This does not apply when overvoltages occur. Overvoltages are all voltages which are above the upper tolerance limit of the rated voltage. They include mainly transient overvoltages which can occur due to atmospheric discharges, but also those due to switching operations or short circuits in power supply grids, and can be galvanically, inductively or capacitively coupled into electronic circuits. Overvoltage protection means have been developed and have been known for more than 20 years to protect electrical or electronic circuits, especially electronic measurement, control and switching circuits, mainly also telecommunications equipment and systems against transient overvoltages wherever they are used.
An important component of an overvoltage protection means of the type under consideration is at least one spark gap which responds at a certain overvoltage, i.e., the sparkover voltage, and thus, prevents overvoltages which are larger than the sparkover voltage of the spark gap from occurring in the circuit which is protected by an overvoltage protection means.
It was stated at the beginning that the overvoltage protection means of the type to which the invention is directed has two electrodes and a breakdown spark gap which acts or exists between the two electrodes. A breakdown spark gap can be an air breakdown spark gap, but can also be a spark gap in which not air, but another gas is found between the electrodes. In addition to overvoltage protection means with a breakdown spark gap, there are overvoltage protection means with a flashover spark gap in which a creeping discharge occurs upon when it responds.
Overvoltage protection means with a breakdown spark gap, compared to overvoltage protection means with a flashover spark gap, have the advantage of a higher surge current carrying capacity, but the disadvantage of a higher and not especially constant sparkover voltage. Therefore, different overvoltage protection means with a breakdown spark gap have already been suggested and have been improved with respect to the sparkover voltage. Here, in the area of the electrodes or the breakdown spark gap which acts between the electrodes, ignition aids have been implemented, for example, such that, between the electrodes, there has been at least one ignition aid which triggers a creeping discharge and which projects at least partially into the breakdown spark gap, which is made in the manner of a crosspiece and which is made of plastic (compare German Patent Application DE 41 41 681 A1 or German Patent Application DE 44 02 615 A1 and corresponding U.S. Pat. No. 5,604,400).
The aforementioned ignition aids which are provided in the known overvoltage protection means can likewise be called “passive ignition aids”, therefore “passive ignition aids” because they do not respond “actively” themselves, but respond only by an overvoltage which occurs on the main electrodes.
German Patent Application DE 198 03 636 A1 and corresponding U.S. Pat. No. 6,111,740 disclose an overvoltage protection means with two electrodes, with a breakdown spark gap which acts between the two electrodes, and an ignition aid. In this overvoltage protection means, the ignition aid is made as an “active ignition aid,” specifically in that, in addition to the two electrodes, called the main electrodes there, there are two more ignition electrodes. These two ignition electrodes form a second breakdown spark gap which is used as an ignition spark gap. In this overvoltage protection means, the ignition aid includes not only the ignition spark gap, but also an ignition circuit with an ignition switching device. When there is an overvoltage on the overvoltage protection means, the ignition circuit with the ignition switching device provides for response of the ignition spark gap. The two ignition electrodes are arranged with respect to the two main electrodes such that, because the ignition spark gap has responded, the breakdown spark gap between the two main electrodes also responds.
In the known, above described embodiment of overvoltage protection means with ignition aids, the ignition aids lead to an improved, specifically lower and more constant sparkover voltage.
In overvoltage protection means of the type under consideration—with or without using an ignition aid—when the breakdown spark gap ignites, the resulting arc forms a low-impedance connection between the two electrodes. First of all, the transient surge current to be diverted flows intentionally by way of this low-impedance connection. However, when the grid voltage is present, then an unwanted grid follow current follows by way of the low-impedance connection of the overvoltage protection means, so that an effort is made to extinguish the arc as quickly as possible after the completed diversion process. One possibility for achieving this object is to increase the arc length, and thus, the arc voltage after the spark gap responds.
Another possibility for extinguishing the arc after the diversion process consists in cooling the arc by the cooling action of insulation walls and the use of insulators which release gas. Here, a strong flow of the extinguishing gas is necessary; this requires high construction effort.
If the arc is extinguished in overvoltage protection means of the type under consideration, first of all, the low-impedance connection between the two electrodes is broken, the space between the two electrodes, i.e., the arc chamber, is however still almost completely filled with plasma. The plasma which is present reduces the sparkover voltage between the two electrodes such that, at the prevailing operating voltage, re-ignition of the breakdown spark gap can occur. This problem occurs especially when the overvoltage protection means has an encapsulated or only half-open housing, since then cooling or volatilization of the plasma is prevented by the essentially closed housing.
To prevent re-ignition of the overvoltage protection means, i.e., the breakdown spark gap, in the past, various measures were taken to drive the ionized gas cloud away from the ignition electrodes or to cool it. However, here, it must be considered that the hot plasma may not simply be blown out of the housing, since otherwise adjacent system parts can be destroyed and there is the danger that individuals located in the vicinity could be injured by the hot gases which are flowing out under high pressure. Therefore, known overvoltage protection means often have several chambers into which the hot plasma can escape after ignition or it is actively driven by blowing. Then, the plasma can cool in the chambers. One disadvantage of these overvoltage protection means, which have the corresponding chambers, consists in that, if the chambers are to be completely closed, very large volumes are necessary so that the dimensions of the overvoltage protection means greatly increase overall. 